Semiconductor device and method for fabricating the same

ABSTRACT

A semiconductor device includes an interfacial layer on a substrate and agate structure on the interfacial layer. Preferably, the gate structure includes a patterned high-k dielectric layer, the patterned high-k dielectric layer comprises a metal oxide layer, and a horizontal direction width of the patterned high-k dielectric layer and a horizontal direction width of the interfacial layer are different. The semiconductor device also includes a first spacer adjacent to the gate structure and on part of the interfacial layer and contacting a top surface of the interfacial layer and a second spacer on the sidewalls of the first spacer and the interfacial layer. Preferably, a planar bottom surface of the second spacer is lower than a planar bottom surface of the first spacer and extending along a same direction as the planar bottom surface of the first spacer.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation application of U.S. patent application Ser. No.14/324,092, filed on Jul. 4, 2014, and all benefits of such earlierapplication are hereby claimed for this new continuation application.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The invention relates to a method for fabricating semiconductor device,and more particularly, to a method of retaining interfacial layer whilea stacked structure is patterned to form gate structure.

2. Description of the Prior Art

In current semiconductor industry, polysilicon has been widely used as agap-filling material for fabricating gate electrode ofmetal-oxide-semiconductor (MOS) transistors. However, the conventionalpolysilicon gate also faced problems such as inferior performance due toboron penetration and unavoidable depletion effect which increasesequivalent thickness of gate dielectric layer, reduces gate capacitance,and worsens driving force of the devices. In replacing polysilicongates, work function metals have been developed to serve as a controlelectrode working in conjunction with high-K gate dielectric layers.

However, in current fabrication of high-k metal transistor, particularlyduring the stage when spacer is formed on the sidewall of gatestructure, issues such as over-etching or undercut often arise andcausing etching gas to etch through spacer until reaching the bottom ofthe gate structure. This induces erosion in high-k dielectric layerand/or bottom barrier metal (BBM) and affects the performance of thedevice substantially. Hence, how to resolve this issue has become animportant task in this field.

SUMMARY OF THE INVENTION

According to an embodiment of the present invention, a semiconductordevice includes an interfacial layer on a substrate and a gate structureon the interfacial layer. Preferably, the gate structure includes apatterned high-k dielectric layer, the patterned high-k dielectric layercomprises a metal oxide layer, and a horizontal direction width of thepatterned high-k dielectric layer and a horizontal direction width ofthe interfacial layer are different. The semiconductor device alsoincludes a first spacer adjacent to the gate structure and on part ofthe interfacial layer and contacting a top surface of the interfaciallayer and a second spacer on the sidewalls of the first spacer and theinterfacial layer. Preferably, a planar bottom surface of the secondspacer is lower than a planar bottom surface of the first spacer andextending along a same direction as the planar bottom surface of thefirst spacer.

These and other objectives of the present invention will no doubt becomeobvious to those of ordinary skill in the art after reading thefollowing detailed description of the preferred embodiment that isillustrated in the various figures and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-5 illustrate a method for fabricating semiconductor deviceaccording to a preferred embodiment of the present invention.

DETAILED DESCRIPTION

Referring to FIGS. 1-5, FIGS. 1-5 illustrate a method for fabricatingsemiconductor device according to a preferred embodiment of the presentinvention. As shown in FIG. 1, a substrate 12, such as a wafer orsilicon-on-insulator (SOI) substrate is provided, in which a pluralityof shallow trench isolations (STIs) 14 are formed in the substrate 12.An interfacial layer 16 is then deposited on the substrate 12 and theSTI 14, and a stack structure 18 is formed on the substrate 12thereafter. The formation of the stack structure 18 is accomplished bysequentially forming a high-k dielectric layer 20, a bottom barriermetal (BBM) layer 22, a silicon layer 24, and a hard mask 26 on theinterfacial layer 16.

In this embodiment, the interfacial layer 16 is preferably composed ofsilicon material such as silicon dioxide (SiO₂), silicon nitride (SiN),or silicon oxynitride (SiON), or other dielectric material with highpermittivity or dielectric constant. The silicon layer 24 is preferablycomposed of single crystal silicon, doped polysilicon, or amorphouspolysilicon, and the hard mask 16 could be selected from the groupconsisting of SiC, SiON, SiN, SiCN and SiBN, but not limited thereto.Despite the hard mask 26 in this embodiment is preferably asingle-layered hard mask, a composite hard mask composed of both siliconnitride layer and silicon oxide layer could also be utilized accordingto the demand of the process, which is also within the scope of thepresent invention.

As the present embodiment pertains to a high-k first process from gatelast process, the high-k dielectric layer 20 preferably has a “I-shaped”cross section and preferably be selected from dielectric materialshaving dielectric constant (k value) larger than 4. For instance, thehigh-k dielectric layer 20 may be selected from hafnium oxide (HfO₂),hafnium silicon oxide (HfSiO₄), hafnium silicon oxynitride (HfSiON),aluminum oxide (Al₂O₃), lanthanum oxide (La₂O₃), tantalum oxide (Ta₂O₅),yttrium oxide (Y₂O₃), zirconium oxide (ZrO₂), strontium titanate oxide(SrTiO₃), zirconium silicon oxide (ZrSiO₄), hafnium zirconium oxide(HfZrO₄), strontium bismuth tantalate (SrBi₂Ta₂O₉, SBT), lead zirconatetitanate (PbZr_(x)Ti_(1-x)O₃, PZT), barium strontium titanate(Ba_(x)Sr_(1-x)TiO₃, BST) or a combination thereof.

In this embodiment, the high-k dielectric layer 20 may be formed byatomic layer deposition (ALD) process or metal-organic chemical vapordeposition (MOCVD) process, but not limited thereto.

Next, as shown in FIG. 2, a patterned mask, such as a patterned resist(not shown) is formed on the hard mask 26, and a pattern transferprocess is conducted by using the patterned resist as mask to partiallyremove the hard mask 26, silicon layer 24, BBM layer 22, and high-kdielectric layer 20 not covered by the patterned resist through singleor multiple etching processes for forming a gate structure 28. In otherwords, the gate structure 28 preferably composed of a patterned high-kdielectric layer 20, a patterned BBM layer 22, a patterned silicon layer24, and a patterned hard mask 26.

Next, as shown in FIG. 3, a spacer formation is conducted by firstforming a liner 32 on the interfacial layer 16 and the gate structure28. The liner 32 is preferably composed of silicon dioxide or siliconnitride, but not limited thereto.

As shown in FIG. 4, an etching back process is then conducted by usingsingle or multiple etching processes to remove part of the liner 32 andpart of interfacial layer 16 for forming a spacer 34 on the sidewall ofthe gate structure 28. According to a preferred embodiment of thepresent invention, the spacer 34 preferably sits on the remaininginterfacial layer 16 and as part of the interfacial layer 16 is removedwith the liner 32 in the aforementioned etching back process, an edge ofthe spacer 34 is aligned with an edge of the interfacial layer 16. Thiscompletes the fabrication of a semiconductor device according to apreferred embodiment of the present invention. Next, a light ionimplantation process could be conducted to form a lightly doped drain(LDD) 30 in the substrate 12 adjacent to two sides of the spacer 34. Theions implanted during the light ion implantation process could beadjusted depending on the type of the transistor. being fabricated Forinstance, if a NMOS transistor were to be fabricated, n-type dopantscould be implanted into the substrate where as if a PMOS transistor wereto be fabricated, p-type dopants could be implanted into the substrate.It should be noted that despite the LDD 30 is formed in the substrate 12after the fabrication of spacer 34 is completed, the LDD 30 could alsobe formed before the spacer 34 is fabricated, which is also within thescope of the present invention.

Referring again to FIG. 4, which illustrates a semiconductor devicestructure according to an embodiment of the present invention. Thesemiconductor device preferably includes a substrate 12, an interfaciallayer 16 on the substrate 12, a gate structure 28 on the interfaciallayer 16, and a spacer 34 adjacent to the gate structure 28 and on partof the interfacial layer 16. As shown in the figure, the gate structure28 includes a patterned high-k dielectric layer 20, a patterned BBMlayer 22 on the high-k dielectric layer 20, a patterned silicon layer 24on the patterned BBM layer 22, and a patterned hard mask 26 on thepatterned silicon layer 24.

Preferably, the interfacial layer 16 is composed of silicon dioxide, thepatterned BBM layer 22 is composed of TiN, the patterned silicon layer24 is composed of polysilicon or amorphous silicon, and the spacer 34 iscomposed of silicon oxide or silicon nitride. Regarding the position ofthe interfacial layer 16 relative to the entire gate structure 28, thewidth of the interfacial layer 16 is preferably wider than the overallwidth of the gate structure 28, and an edge of the interfacial layer 16is aligned with an edge of the spacer 34.

After the spacer 34 is fabricated, as shown in FIG. 5, typicaltransistor fabrication process could be carried out by forming a mainspacer 48 on the sidewall of the spacer 34, and then forming asource/drain region 36 in the substrate 12 adjacent to two sides of themain spacer 48. Next, a contact etch stop layer (CESL) 38 could beformed on the gate structure 28, and an interlayer dielectric (ILD)layer 40 could be formed on the CESL 38. It should be noted thatelements such as epitaxial layer and silicides could also be formedbefore the CESL 38, and as the formation of these elements are wellknown to those skilled in the art, the details of which are notexplained herein for the sake of brevity.

Next, a replacement metal gate (RMG) process could be conducted totransform the gate structure 28 into a metal gate. The RMG process couldbe accomplished by performing a selective dry etching or wet etchingprocess, such as using etchants including ammonium hydroxide (NH₄OH) ortetramethylammonium hydroxide (TMAH) to remove the silicon layer 24 inthe gate structure 28 for forming a recess (not shown). Next, aconductive layer 46 including a U-shaped work function metal layer 42and low resistance metal layer 44 is deposited into the recess, andanother planarizing process is conducted thereafter to form a metalgate.

In this embodiment, the work function metal layer 42 is formed fortuning the work function of the metal gate so that the device could beadapted in an NMOS or a PMOS transistor. For an NMOS transistor, thework function metal layer 42 having a work function ranging between 3.9eV and 4.3 eV may include titanium aluminide (TiAl), zirconium aluminide(ZrAl), tungsten aluminide (WAl), tantalum aluminide (TaAl), hafniumaluminide (HfAl), or titanium aluminum carbide (TiAlC), but is notlimited thereto. For a PMOS transistor, the work function metal layer 42having a work function ranging between 4.8 eV and 5.2 eV may includetitanium nitride (TiN), tantalum nitride (TaN), tantalum carbide (TaC),but is not limited thereto. A barrier layer (not shown) could be formedbetween the work function metal layer 42 and the low resistance metallayer 44, in which the material of the barrier layer may includetitanium (Ti), titanium nitride (TiN), tantalum (Ta) or tantalum nitride(TaN). Furthermore, the material of the low resistance metal layer 44may include copper (Cu), aluminum (Al), tungsten (W), titanium aluminum(TiAl), cobalt tungsten phosphide (CoWP) or any combination thereof.

Overall, the present invention utilizes an etching process to onlyremove part of the hard mask, silicon layer, BBM layer, and high-kdielectric layer while leaving the interfacial layer intact during theprocess for patterning a stack structure into a gate structure. Byfollowing this approach, the spacer formed thereafter would be sittingon part of the interfacial layer and as the width of the interfaciallayer becomes wider than the overall width of the gate structure, theextended portion of the interfacial layer could be used to increase thestructural strength of the bottom portion of the gate structure.Ultimately, the high-k dielectric layer and/or BBM layer situating inthe bottom of the gate structure are protected from the etching gas usedduring the spacer formation process.

Those skilled in the art will readily observe that numerousmodifications and alterations of the device and method may be made whileretaining the teachings of the invention. Accordingly, the abovedisclosure should be construed as limited only by the metes and boundsof the appended claims.

What is claimed is:
 1. A semiconductor device, comprising: aninterfacial layer on a substrate; a gate structure on the interfaciallayer, wherein the gate structure comprises a patterned high-kdielectric layer, the patterned high-k dielectric layer comprises ametal oxide layer, and a horizontal direction width of the patternedhigh-k dielectric layer and a horizontal direction width of theinterfacial layer are different, wherein the horizontal direction widthof the patterned high-k dielectric layer is taken in a horizontaldirection from the substrate; a first spacer adjacent to the gatestructure and on part of the interfacial layer and contacting a topsurface of the interfacial layer; and a second spacer on the sidewallsof the first spacer and the interfacial layer, wherein a planar bottomsurface of the second spacer is lower than a planar bottom surface ofthe first spacer and extending along a same direction as the planarbottom surface of the first spacer.
 2. The semiconductor device of claim1, wherein the interfacial layer comprises silicon dioxide.
 3. Thesemiconductor device of claim 1, wherein the gate structure comprisesthe patterned high-k dielectric layer, a patterned bottom barrier metal(BBM) layer on the patterned high-k dielectric layer, a patternedsilicon layer on the patterned BBM layer, and a patterned hard mask onthe patterned silicon layer.
 4. The semiconductor device of claim 3,wherein the patterned BBM layer comprises TiN.
 5. The semiconductordevice of claim 3, wherein the patterned silicon layer comprisesamorphous silicon or polysilicon.
 6. The semiconductor device of claim1, wherein the first spacer comprises silicon dioxide or siliconnitride.
 7. The semiconductor device of claim 1, wherein the horizontaldirection width of the interfacial layer is wider than a horizontaldirection width of the gate structure.
 8. The semiconductor device ofclaim 1, wherein an edge of the interfacial layer is aligned with anedge of the first spacer.
 9. The semiconductor device of claim 1,further comprising a contact etch stop layer (CESL) around the secondspacer, wherein a bottom surface of the CESL is lower than the planarbottom surface of the first spacer.